Inclusion Criteria:

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•  Females >18 years old.
• Pre- or post-menopausal HR+ metastatic breast cancer patients who are:
  • • Group 1:  scheduled to initiate 1st line combination therapy with an FDA-approved CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) and an FDA-approved endocrine therapy for pre- or post-menopausal HR+ mBC.
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    • Group 2: scheduled to initiate second or later lines of therapy with an FDA-approved CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) with or without an FDA-approved endocrine therapy for pre- or post-menopausal HR+ mBC.
    • Group 3: currently receiving 1st line therapy with an FDA-approved CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) and an FDA-approved endocrine therapy for pre- or post-menopausal HR+ mBC.
    • Group 4:  scheduled to initiate, or are currently receiving, 1st or 2nd line therapy of an FDA-approved endocrine therapy (single agent) for pre- or post-menopausal HR+ mBC.
    • NOTE:  Patients that meet eligibility criteria for any of the above groups that are already enrolled in an on-going clinical trial are eligible for co-enrollment to this observational study.

Exclusion Criteria: 

• Treatment including investigational agent or therapies.
• Early breast cancer diagnosis.
• Male breast cancer.
• Currently receiving treatment for other active malignancy at time of registration EXCEPTIONS: Non-melanoma skin cancer or carcinoma-in-situ (e.g. of cervix, prostate).
• Inability to give written informed consent. 

Note: Other protocol defined Inclusion/Exclusion criteria may apply.

Eligibility last updated 9/19/23. Questions regarding updates should be directed to the study team contact